A synthetic antiferromagnet structure designates a structure composed of a plurality of ferromagnetic layers and an intermediate non-magnetic layer(s) interposed therebetween. The synthetic antiferromagnet structure is configured so as to antiferromagnetically couple the adjacent ferromagnetic layers. More specifically, each intermediate non-magnetic layer is formed of material with a film thickness so that the adjacent ferromagnetic layers are antiferromagnetically coupled. It is well-known to those skilled in the art of the MRAM that the adjacent ferromagnetic layers can be antiferromagnetically coupled by appropriately determining material and the film thickness of the intermediate non-magnetic layer.
For example, as disclosed in published Japanese translations of PCT international publication No. 2003-536267, the synthetic antiferromagnet structure is often used as a fixed layer (pinned layer) of the MRAM memory cell. One advantage of using the synthetic antiferromagnet structure as a fixed layer of the MRAM memory cell is to decrease an undesired bias magnetic field applied to a free layer of the MRAM memory cell. This results from the fact that the synthetic antiferromagnet structure allows decreasing a total magnetic moment down to ideally zero. Since a magnetic field generated by the synthetic antiferromagnet structure is proportional to the magnitude of the magnetic moment, a magnetic field applied to a free layer from a fixed layer can be decreased by using a synthetic antiferromagnet structure as the fixed ferromagnetic layer.
In recent years, a technique has been studied which uses synthetic antiferromagnet structures as the free layer of the MRAM memory cell. U.S. Pat. No. 6,545,906 discloses a technique for improving the selectivity of memory cells and data retention properties, and further to suppress the shape dependency of a coercive field by applying the synthetic antiferromagnet structure to the free layer.
In order to employ the synthetic antiferromagnet structure in the MRAM memory cell, there is required a technique to direct magnetizations of ferromagnetic layers within the synthetic antiferromagnet structure to desired directions. The fixed layer is required to have magnetizations of the synthetic antiferromagnet structure directed to desired directions in the MRAM manufacturing process. The free layer, on the other hand, is required to have magnetizations of the synthetic antiferromagnet structure directed to a direction corresponding to data to be written to the MRAM memory cell.
When the synthetic antiferromagnet structure is used as the fixed layer, the magnetization directions of ferromagnetic layers within the synthetic antiferromagnet structure are most typically controlled by using an antiferromagnetic layer. FIG. 1 is a cross-sectional view showing a typical structure of the MRAM memory cell in which the synthetic antiferromagnet structure is used as the fixed layer. An MRAM memory cell 1000 includes a fixed layer 1100, a free layer 1200, and a tunnel barrier layer 1300 interposed therebetween. The fixed layer 1100 is formed of a synthetic antiferromagnet structure. More specifically, the fixed layer 1100 is composed of ferromagnetic layers 1101 to 1103 and non-magnetic layers 1111 and 1112 interposed therebetween. The fixed layer 1100 is connected to an antiferromagnetic layer 1400, thereby magnetizations of the ferromagnetic layers 1101 to 1103 are fixed.
The magnetization directions of the ferromagnetic layers 1101 to 1103 are controlled by annealing while applying a strong external magnetic field. Specifically, a strong magnetic field is applied to the MRAM memory cell 1000 so as to align the magnetizations of the ferromagnetic layers 1101 to 1103 in the same direction. When the external magnetic field has been removed, the magnetization of one of the ferromagnetic layers 1101 to 1103 is reversed, thereby the magnetizations of the ferromagnetic layers 1101 to 1103 are rearranged to directions so that the synthetic antiferromagnet structure is stabilized in terms of energy. In this rearrangement, the magnetization of the ferromagnetic layer 1103 is not reversed because a magnetization reversal thereof is prevented by an exchange interaction from the antiferromagnetic layer 1400; the magnetization of the ferromagnetic layer 1103 is fixed to the direction of the external magnetic field. Accordingly, in order to stabilize the synthetic antiferromagnet structure in terms of energy, the magnetization of the ferromagnetic layer 1102 is reversed to the direction opposite to the external magnetic field, and the magnetization of the ferromagnetic layer 1101 is directed to the same direction with the external magnetic field without being reversed. This implies that the magnetizations of the ferromagnetic layers 1101 to 1103 are directed to the desired directions by applying the external magnetic field to a direction to which the magnetizations of the ferromagnetic layers 1101 and 1103 are to be directed.
In the magnetization direction control mentioned above, the antiferromagnetic layer 1400 is coupled to the synthetic antiferromagnet structure so as to fix final magnetization directions of the ferromagnetic layers 1101 to 1103. The antiferromagnetic layer 1400 plays an important role in controlling the magnetization directions.
On the other hand, U.S. Pat. No. 6,545,906 discloses a method for reversing magnetization directions of ferromagnetic layers within the synthetic antiferromagnet structure to the desired directions when the synthetic antiferromagnet structure is used as the free layer, which is specifically a method for writing data to the synthetic antiferromagnet structure. This U.S. Pat. No. 6,545, 906 discloses two methods for writing data to the synthetic antiferromagnet structure: the toggle writing method and the direct writing method.
FIG. 2 is a cross sectional view showing a structure of an MRAM memory array disclosed in U.S. Pat. No. 6,545,906, and FIG. 3 is a plane view showing a structure of the above MRAM memory array. As shown in FIG. 2, the MRAM memory array includes a word line 2400, a bit line 2500, and a memory cell 2000 interposed therebetween. The memory cell 2000 is composed of a fixed layer 2100, a free layer 2200, and a tunnel barrier layer 2300 interposed therebetween. The fixed layer 2100 and the free layer 2200 are both composed of the synthetic antiferromagnet structure. The fixed layer 2100 includes ferromagnetic layers 2101 and 2102, and a non-magnetic layer 2103 interposed therebetween, while the free layer 2200 includes ferromagnetic layers 2201 and 2202, and a non-magnetic layer 2203 interposed therebetween.
As shown in FIG. 3, easy axes of the ferromagnetic layers 2201 and 2202 are directed at an angle of 45 degrees to both the word line 2400 and the bit line 2500. A magnetization M1, of the ferromagnetic layer 2201 and a magnetization M2 of the ferromagnetic layer 2202 are antiparallelly directed each other along the direction of the easy axes.
FIGS. 4 and 5 show a procedure of the toggle writing method disclosed in U.S. Pat. No. 6,545,906. Referring to FIG. 5, it should be noted that an x-y coordinate system is introduced below for convenience of explanation. In the x-y coordinate system, the direction of the x-axis is defined as being parallel to the word line 2400, and the direction of the y-axis is defined as being parallel to the bit line 2500.
In the toggle writing method, as shown in FIG. 4, a current IWL is applied to the word line 2400 in the +x direction at the time t1. As shown in FIG. 5, the application of the current IWL generates a magnetic field HWL in the +y direction, and the magnetic field HWL causes the magnetization M1 of the ferromagnetic layer 2201 and the magnetization M2 of the ferromagnetic layer 2202 to be slightly rotated in the +y direction. The rotation angle of the magnetizations M1 and M2 is an angle in which a resultant magnetization MR thereof is in parallel with the +y direction.
Subsequently, a current IBL is applied to the bit line 2500 in the +x direction at a time t2. The application of the current IBL generates a resultant magnetic field HWL+HBL at an angle of 45 degrees to both the x-axis and the y-axis. This resultant magnetic field HWL+HBL causes the magnetizations M1 and M2 to be rotated clockwise so that the resultant magnetization MR is directed in parallel to the resultant magnetic field HWL+HBL.
This is followed by shutting off the current IWL applied to the word line 2400 at the time t3. The shutoff of the current IWL causes only the magnetic field HBL to be applied to the ferromagnetic layers 2201 and 2202 in the +x direction. This magnetic field HBL causes the magnetizations M1 and M2 to be further rotated clockwise so that the resultant magnetization MR is directed in parallel to the magnetic field HBL.
Finally, the current IBL applied to the bit line 2500 is shut off at the time t4. The shutoff of the current IBL results in that the magnetic fields stop being applied to the magnetizations M1 and M2, and the magnetizations M1 and M2 are redirected along the closest easy axis. As a result, the magnetization M1 and M2 are reversed to the direction opposite to the original direction.
One feature of the toggle writing method is that the magnetizations M1 and M2 are reversed every time the toggle writing is performed, regardless of the original directions of the magnetizations M1 and M2.
On the other hand, the direct writing method is a method in which it depends on the original direction of the magnetizations M1 and M2 whether or not the magnetizations M1 and M2 are reversed, as shown in FIGS. 6 and 7. The direct writing method controls the reversal of the magnetizations M1 and M2 by using the resultant magnetization MR that initially exists in the synthetic antiferromagnet structure due to the unbalance of the magnetizations M1 and M2. More specifically, as shown in FIG. 6, the application of the magnetic field HWL causes the magnetizations M1 and M2 to be largely rotated clockwise, if the direction of the magnetic field HWL generated by the application of the current IWL forms an obtuse angle to the direction of the resultant magnetic field MR with no magnetic field applied. In this case, the magnetizations M1 and M2 to be reversed when the magnetic field HBL is then applied. On the other hand, as shown in FIG. 7, the magnetizations M1 and M2 is not reversed, if the direction of the magnetic field HWL forms an acute angle to the direction of the resultant magnetization MR with no magnetic field applied. It should be noted that the initial existence of the resultant magnetization MR is important in the direct writing method.
There is room for improvement in the above-mentioned prior arts in the following points. The technique to control magnetizations of ferromagnetic layers in the synthetic antiferromagnet structure using the antiferromagnetic layer suffers from a problem of the diffusion of materials of the antiferromagnetic layer. Anti-ferromagnetic material used in the MRAM generally includes manganese (Mn); however, manganese within the antiferromagnetic material diffuses in a heat treatment at a high temperature. The diffusion of manganese undesirably causes deterioration of the characteristics of the MRAM memory cell.
The toggle writing method disclosed in U.S. Pat. No. 6,545,906, on the other hand, suffers from a drawback that a read operation is required before performing a write operation. The toggle writing results in reversal of the magnetizations of the ferromagnetic layers within the synthetic antiferromagnet structure, regardless of the original states. Accordingly, in order to direct the magnetizations toward the desired directions, magnetization directions of the ferromagnetic layers of the synthetic antiferromagnet structure are determined before writing data to the synthetic antiferromagnet structure, which implies the read operation needs to be executed. Therefore, the toggle writing operation is essentially unsuitable for controlling the magnetizations of the ferromagnetic layers of the synthetic antiferromagnet structure used as the fixed layer. When the synthetic antiferromagnet structure is used as a free layer, the fact that the read operation needs to be executed before a write operation is not preferable, because the write cycle time is increased.
In addition, the direct writing method disclosed in U.S. Pat. No. 6,545,906 suffers from a drawback that the magnetizations of the synthetic antiferromagnet structure are not essentially balanced, that is, the initial existence of the resultant magnetization is required. The existence of the resultant magnetization implies that the synthetic antiferromagnet structure has a magnetic moment and the synthetic antiferromagnet structure generates the magnetic field accordingly. This causes magnetic interference to be generated between adjacent MRAM memory cells. This is not preferable because the advantages of the synthetic antiferromagnet structure are reduced in a sense.
Japanese Laid Open Patent Applications Nos. 2003-8100 and 2003-309305 disclose other techniques to utilize the synthetic antiferromagnet structure as the free layer. However, these techniques are aimed to improve a reproduced output of a magnetic head. These documents do not refer to anything about an MRAM data writing or a magnetization direction control of the fixed layer.
In addition, Japanese Laid Open Patent Application No. 2003-16624 discloses a magnetic recording medium including a synthetic antiferromagnet structure. However, this document does not disclose anything about the MRAM data writing or the magnetization direction control of the fixed layer.
From the background as described above, there is a need for providing a new technique for the control of the magnetization direction of ferromagnetic material within a synthetic antiferromagnet structure.